Pressure swing adsorption process

ABSTRACT

In air separation and other gas separations employing pressure swing adsorption technology, reduced cycle times are achieved by an advantageous combination of simultaneous processing steps. The repressurization step in which the upper adsorption pressure is reached in each adsorbent bed is carried out while simultaneously withdrawing product gas from that bed. Following cocurrent depressurization steps in which released void space gas is used for pressure equalization and provide purge gas purposes, each bed, in turn, is further cocurrently depressurized, and the released gas is used for pressure equalization purposes, while the bed is simultaneously being countercurrently depressurized. This latter countercurrent depressurization is then continued to a lower desorption pressure is reached, after which the bed is partially repressurized by pressure equalization, before being further repressurized to upper adsorption pressure with simultaneous withdrawal of product as the next processing cycle is begun.

This application is a Division of prior U.S. application Ser. No.658,306, filing date Oct. 5, 1984, now U.S. Pat. No. 4,589,888 issuedMay 20, 1986.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates the purification of gases in a pressure swingadsorption system. More particularly, it relates to improvements in theprocessing cycle and system enabling improved performance to beachieved.

2. Description of the Prior Art

The pressure swing adsorption (PSA) process provides a commerciallydesirable technique for separating and purifying at least one componentof a feed gas mixture containing said component and at least oneselectively adsorbable component. Adsorption occurs in an adsorbent bedat a higher adsorption pressure, with the selectively adsorbablecomponent thereafter being desorbed by reducing the adsorbent bedpressure to a lower desorption pressure. The carrying out of the PSAprocess in multi-bed systems is illustrated by the Wagner patent, U.S.No. 3,430,418, relating to a system having a least four beds. As isgenerally known and described in this patent, the PSA process iscommonly carried out, on a cyclic basis, in a processing sequence thatincludes, in each bed, (1) higher pressure adsorption with release ofproduct effluent from the product end of the bed, (2) cocurrentdepressurization to intermediate pressure with release of void space gasfrom the product end thereof, (3) countercurrent depressurization to alower desorption pressure, (4) purge and (5) repressurization. The voidspace gas released during the cocurrent depressurization step iscommonly employed for pressure equalization purposes and to providepurge gas to a bed at its lower desorption pressure.

In a variation of said PSA processing described above with reference tosystems having four or more absorbent bends, a conventional three bedsystem was devised for use in the separation and recovery of air andother such separations. This system was based on the increasing pressureadsorption step described in the McCombs patent, U.S. No. 3,738,087. Inone embodiment thereof, air is added to an adsorbent bed for therepressurization thereof, with nitrogen being selectively adsorbed andwith oxygen being discharged from the product end of the bed at ratessuch that the bed pressure increases to upper adsorption pressure. A PSAcycle incorporating said increasing pressure adsorption step includes(1) said increasing pressure adsorption step, (2) cocurrentdepressurization to intermediate pressure with release of void space gasfrom the product end thereof, (3) countercurrent depressurization to alower desorption pressure, (4) purge and (5) partial repressurization.The void space gas released during the cocurrent depressurization stepis employed, in this embodiment, for passage to other beds in the systemin a pressure equalization-provide purge-pressure equalization sequence.This latter cycle makes unnecessary a constant pressure adsorption stepas employed in the Wagner cycle. This enables more time for bedregneration, i.e. countercurrent depressurization and purge, within agiven cycle time so as to enable greater productivity and recoveryand/or purity to be obtained from a given system, particularly insystems designed for relatively short overall cycle time operation.

Using such a three bed system with each bed containing commercial 13X,8×12 bead form, molecular sieve in air separation operations, an oxygenrecovery of 48% and a productivity (BSF) of 4,000 lb. 13X molecularsieve per one ton per day (TPD) of oxygen have been obtained. Saidrecovery is defined as the percent or volume fraction of the feed airoxygen removed from the feed stream and delivered as oxygen product.Productivity is defined as the pounds of molecular sieve required togenerate 1 TPD of contained oxygen. The recovery and productivity valuesreferred to above were obtained on the basis of a 180 second total cycletime for the 3-bed PSA system, with feed air being introduced at amaximum pressure of 40 psig, with product being discharged at 20 psig.

While such standard 3-bed system is desirable for various commercialapplications, there is, nevertheless, a desire in the art to improveproduct recovery and productivity. Difficulties have been encountered,however, in achieving such objectives. Thus, the total cycle time had tobe reduced to less than said 180 seconds to yield a significant BSFreduction (productivity increase) compared to said standard 3-bedoperation. However, reductions in individual step times, i.e. the purgeand pressure equalization steps, are limited by gas velocity and bedfluidization limits, or by applicable cycle performance standards. Suchlimitations prevent the achieving of substantial cycle time reductionsonly by means of reductions in the duration of the individual cyclesteps. With respect to the standard 4-bed system, on the other hand, theaddition of a fifth adsorbent bed to increase single bed capacity limitsby means of standard cycling techniques applicable to said systems wouldnecessarily result in an increase in total cycle times and in the BSFvalues for any given application. Such an increase in BSF value wouldcompromise any potential increase in productive capacity derived from anincrease in the number of vessels employed in the PSA system. Inaddition, size limitations on PSA-oxygen adsorbent beds limit themaximum capacity of a single PSA train, so that the development of meansto reduce the BSF would be required to increase the maximum capacitylimits of such a single PSA train. There remains in the art, therefore,a need to develop improvements in the PSA art enabling reductions in BSFand increased single train capacity to be achieved. Such improvementsadvantageously would enable the overall cycle time to be reduced, whileenabling sufficient time to complete each individual cycle time withoutdegradation of product purity or recovery.

It is an object of the invention, therefore, to provide an improved PSAprocess and system.

It is another object of the invention to provide a PSA process andsystem for the enhanced separation and recovery of oxygen from air.

It is another object of the invention to provide a PSA process andsystem enabling overall cycle times to be minimized while enablingsufficient time to complete each individual cycle step withoutdegradation in product purity or recovery.

With these and other objects in mind, the invention is hereinafterdescribed in detail, the novel features thereof being particularlypointed out in the appended claims.

SUMMARY OF THE INVENTION

The PSA process of the invention, enabling product recovery and/orpurity to be enhanced, includes cocurrent depressurization of each bedfrom upper adsorption pressure with released void space gas being usedfirst for pressure equalization and then for provide purge purposes witha three or more bed system. Cocurrent depressurization is then continuedfor pressure equalization purposes, while the bed is simultaneouslycountercurrently depressurized. Upon completion of this pressureequalization step, the bed is further countercurrently depressurized,purged and partially repressurized by pressure equalization with anotherbed. Final repressurization to upper adsorption pressure is accomplishedwhile discharging product gas from the bed, obviating the need foremploying a constant pressure adsorption step at the upper adsorptionpressure. The time required for the separate provide purge step isdesirably shorter than the purge step, with the overall countercurrentdepressurization and purge time enabling adequate bed regeneration to beaccomplished with the overall shorter total cycle times permissible inthe practice of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The objects of the invention are accomplished by the incorporation intothe PSA process and system as described above with reference to theWagner patent of simultaneous cycle steps relating to the cocurrentdepressurization and the countercurrent depressurization portions of theoverall cycle. Such simultaneous steps enable the total cycle time to bereduced and BSF reductions, i.e. increased adsorbent productivity, to beobtained. Enhanced bed capacity can be achieved in the practice of theinvention at minimized overall cycle times without degradation ofproduct effluent purity or recovery. The invention has been found toactually result in significant improvements in product recovery ascompared with the standard commercial 3-bed PSA process and systemreferred to above.

It should be noted that the invention, in a desirable embodiment, isbased upon the use of a constant adsorption pressure step, as in theWagner cycle referred to above, wherein feed gas is passed to anadsorption bed maintained at an upper adsorption pressure level, withthe more readily adsorbable component being selectively adsorbed andwith the less readily adsorbable component being discharged from theproduct end of the bed as product effluent. Such constant adsorptionpressure cycles, modified in accordance with the invention, enable lowerBSF requirements to be achieved, which result in higher single traincapacities and lower adsorbent inventory requirements for a givenproduct capacity and purity. The following description of the inventionwill be understood, therefore, to represent a desirable modification ofthe overall processing cycle of Wagner as recited above.

The invention can advantageously be employed in multi-bed PSA systemshaving at least four adsorbent beds therein, with systems containingfour beds being highly desirable for some applications. Five, six orseven adsorbent bed systems are also desirable in some instances. Whilethe invention can be practiced in systems having 8 or more beds, it iscommonly expedient to employ two 4-bed systems, or the like, as analternative to such higher number of beds in a single system. It will beunderstood that in such multi-bed systems, the feed gas may be passed tomore than one bed at any particular stage of the processing cycle. Thus,the feed gas is often passed to at least two beds at any given time inthe operation of such multi-bed systems. As with respect to conventionalpractice, the PSA process desirably employs two, three or more pressureequalization steps in which cocurrent depressurization gas released fromone bed at an elevated pressure is used to partially repressurizeanother bed initially at lower pressure and to provide purge gas to abed at lower desorption pressure. Thus, the invention can be used in avariety of processing cycles such as, for example, those involving fiveor more beds with two beds on adsorption at any given time, inoverlapping sequence, during the overall PSA processing cycle. Thoseskilled in the art will appreciate that various other PSA processes andsystems can be adapted so as to take advantage of the desirable benefitsof the invention in desirable PSA cycles.

The practice of the invention can be illustrated by Table I below withrespect to a four bed embodiment of the invention:

                                      TABLE I                                     __________________________________________________________________________    Bed No.                                                                        1234                                                                               ##STR1##                                                                __________________________________________________________________________

In this Table with respect to each bed, A represents an adsorption stepat an upper adsorption pressure with discharge of the less readilyadsorbable component as product effluent from the product end of thebed; PP represents a cocurrent depressurization step in which void spacegas is released from the product end of the bed for use in providingpurge gas to another bed typically at its lower desorption pressure,said bed being depressurized from an upper intermediate pressure to anintermediate pressure level; P represents a purge step typically atlower desorption pressure in which void space gas released from anotherbed is passed directly to said bed undergoing its purge step, with saidpurge step being seen to be of longer duration than the provide purgestep PP; R represents repressurization to upper adsorption pressure;E1/PP represents one of the novel steps of the invention, namely acocurrent depressurization step in which void space gas released duringcocurrent depressurization from said upper adsorption pressure to anupper intermediate pressure is passed simultaneously to one other bed inthe system being partially repressurized to said upper intermediatepressure and to a second other bed as purge gas for said bed at itslower desorption pressure level; and E2/BD represents the other novelstep wherein further cocurrent depressurization from said intermediatepressure level to a lower intermediate pressure is carried out withrelease of additional void space gas from the product end of the bed,said void space gas being passed to another bed in the system forpressure equalization therebetween at said lower intermediate pressure,while the bed is being simultaneously depressurized countercurrently bythe discharge of gas from the feed end of the bed. The countercurrent,or BD, portion of said E2-BD step is continued after completion of saidpressure qualization down to the lower desorption pressure of the bed.In the process of the embodiment illustrated in said Table I, it will beseen that only one of the four beds in the system is on the adsorptionstep, on a cyclic basis, at any given time in the cycle. As two pressureequalization steps are employed, i.e. said E1/PP and E2/BD and theircounterparts E2 and E1, the overall cycle is referred to in the headingof the Table as a (412) E1/PP-E2/BD cycle, the 4 representing the numberof beds, the 1 representing the number of beds on adsorption at anygiven time, and the 2 representing the number of direct pressureequalization steps, and E1/PP-E2/BD denoting the point of novelty of theinvention wherein the two simultaneous processing features describedabove are employed in the PSA processing cycle to obtain the benefitsreferred to herein.

In the processing cycle illustrated in Table I, the E1/PP step iscarried out, e.g. in bed 1, by the cocurrent depressurization of saidbed with the void space gas released from the product end of the bedbeing simultaneously passed to bed 3 for pressure equalization at upperintermediate pressure and to bed 4 for the initial portion of the purgestep in said bed. Following the continuing cocurrent depressurizationstep wherein void space gas from bed 1 is passed to said bed 4 as purgegas, with bed 1 being depressurized further to an intermediate pressurelevel, the E2/BD step is carried out with additional void space gasbeing released from the product end of bed 1, which is cocurrentlydepressurized to a lower intermediate pressure, said gas being passed tobed 4 for pressure equalization at said lower intermediate pressure. Bed1 is simulataneously depressurized countercurrently by the discharge ofgas from the feed end thereof. The BD portion of the step is continuedafter completion of the E2 portion upon pressure equalization ofdepressurizing bed 1 and repressurizing bed 4 at said lower intermediatepressure. It will be seen from this example that the E2 step representspartial repressurization of a bed from its lower desorption pressure tolower intermediate pressure by the passage of void space gas thereto,typically directly, from a bed being cocurrently depressurized in itsE2/BD step from intermediate pressure to said lower intermediatepressure by pressure equalization with said bed being partiallyrepressurized from its lower desorption pressure to said lowerintermediate pressure. Similarly, E1 represents further partialrepressurization to upper intermediate pressure by the passage of voidspace gas thereto, typically directly, from a bed being cocurrentlydepressurized in its E1/PP step from upper adsorption pressure to upperintermediate pressure by pressure qualization with said bed beingpartially repressurized from its lower to its upper intermediatepressure.

When the invention as represented by the processing cycle of Table I isemployed in a practical commercial air separation operation, each bedbeing operated at an upper adsorption pressure of 40 psig, a 160 secondtotal or overall cycle time can be effectively utilized. The BSF ofcommercial 13X, 8×12 beaded, molecular sieve adsorbent, has 3,000 lb. ofsaid 13X/TPD of oxygen product at 90% product purity. Recovery of oxygenproduct was 53%. By contrast, at the same 40 psig adsorption pressure,using commercial 5A, 8×12 beads and producing product oxygen at said 90%purity, a 4-bed Wagner cycle system required a 240 second total cycletime, with a higher BSF, i.e. lower productivity, of 6,000 lb. of said5A /TPD of oxygen product.

In another desirable embodiment of the system, five adsorbent beds areemployed with two beds on adsorption at all times. As in the previousillustrated embodiment, two pressure equalization steps are employed,together with the E1/PP and E2/BD steps of the invention. Hence, thecycle is designated as a (522) E1/PP-E2/BD cycle in the heading of TableII below.

                                      TABLE II                                    __________________________________________________________________________    Bed                                                                           No.                                                                            12345                                                                            ##STR2##                                                                  __________________________________________________________________________

In the cycle of Table II, A, E1/PP, PP, E2/BD, P, E2, E1 and R all havethe same meanings as were indicated above with respect to the Table Iembodiment. In the practice of said (522) E1/PP-E2/BD Cycle for oxygenrecovery at said 90% purity by air separation, employing an upperadsorption pressure of 40 psig and using the same commercial 13Xmolecular sieve adsorbent, in 8×12 beaded form, a total cycle time of200 seconds is employed, with an BSF of about 3,800 lb. of said BX/TPDof oxygen product. Recovery of product oxygen is 54%. By contrast at thesame 40 psig adsorption pressure, using the same commercial 13X, 8×12beads and producing product oxygen at said 90% purity, a standard 3-bedcycle as described above involves the use of a 180 second total cycletime, with a BSF of 4,000 lb. of said 13X/TPD of oxygen product. Therecovery of oxygen product was only 49%.

Those skilled in the art will appreciate that various changes andmodifications can be made in the details of the PSA process and systemas described herein without departing from the scope of the invention asrecited in the appended claims. It will also be appreciated that PSAsystems necessarily incorporate various conduits, valves and othercontrol features to accomplish the necessary switching of the adsorbentbeds from one processing step to the next in appropriate sequence. Theinvention can readily be employed using conventional conduits andcontrol features well known in the art. For purposes of the invention,the PSA system will comprise conduit means for passing void space gasreleased from the product end of a bed during cocurrent depressurizationfrom said upper intermediate pressure simultaneously to other beds inthe system, said gas being passed to one bed for pressure equalizationat said upper intermediate pressure, and to another bed for providingpurge gas to said bed. Commercially available control means can readilybe employed for enabling the passage of void space gas from the bedbeing cocurrently depressurized to continue until an intermediatepressure level is reached, with the released gas being passed to the bedbeing purged, following termination of the passage of gas to the bedbeing pressure equalized at said upper intermediate pressure. Conduitmeans are also provided for passing additional void space gas releasedfrom the product end of the bed, upon further cocurrent depressurizationthereof from said intermediate pressure to a lower intermediatepressure, to another bed in the system for pressure equalizationtherebetween at lower intermediate pressure and for simultaneouslydischarging gas from the feed end of the bed. Control means can likewisebe provided for precluding the passage of gas from the bed to which gashad been passed during pressure equalization at lower intermediatepressure upon continuance of the discharge of gas from the feed end ofthe bed, i.e. the continued BD portion of the E2/BD step, down to saidlower desorption pressure upon completion of the pressure equalizationstep at lower intermediate pressure. It is convenient to employ anin-line check valve as said latter control means, with said check valvebeing adapted to prevent back-flow of gas at lower intermediate pressureinto said bed being further depressurized from said lower intermediatepressure to lower desorption pressure. Those skilled in the art willappreciate that such variations or modifications of the PSA process andsystem of the invention can include in appropriate circumstances, theinclusion of additional pressure equalization steps or the providing ofadditional adsorbent beds on the adsorption step at any given time inoverlapping processing sequence.

While the invention has been described above with reference to aconstant pressure adsorption step in contrast to the standard 3-bedsystem employing an increasing pressure adsorption step withoutsubsequent adsorption at a constant upper adsorption pressure level, itshould be noted that the invention can be practiced by incorporating anincreasing pressure adsorption step as the repressurization step, i.e.step R of Tables I and II, following partial repressurization of a bedby pressure equalization, i.e. steps E2 and E1 of said Tables. In suchan embodiment, therefore, the repressurization to upper adsorptionpressure is carried out with product effluent being simultaneouslydischarged from the product end of the bed. In such circumstance, itwill be appreciated that increased amounts of product gas can berecovered in any given cycle time without sacrifice of the timeavailable for regeneration of the bed, or alternatively, the constantpressure adsorption step can thus be made shorter to allow for more timefor bed regeneration purposes, thereby enabling product purity and/orrecovery to be enhanced. It should also be noted that said increasingpressure adsorption step can advantageously be employed, together withthe E2/BD step of the invention in 3-bed PSA systems without a constantpressure adsorption step or the E1/PP step of the invention. Thisprocessing variation can also be employed in PSA systems having morethan three beds. Thus, a processing sequence of E1 (depressurizationfrom upper adsorption pressure), PP, E2/BD, P, E2 (partialrepressurization) and increasing pressure adsorption to upper adsorptionpressure could be employed in the practice of this variation.

The pressure swing adsorption process and system herein disclosed andclaimed can be advantageously employed to selectively adsorb at leastone component of a feed gas mixture, thereby separating and purifying adesired product effluent gas. While the invention is particularlyadvantageous for separating and recovering oxygen as the less readilyadsorbable component of air from nitrogen as the more readily adsorbablecomponent thereof, it will be appreciated by those skilled in the artthat various other separations, including the recovery of hydrogen fromfeed gas mixtures or even the separation and recovery of nitrogen as theproduct effluent from feed air is feasible depending upon theperformance characteristics of the particular adsorbent employed in thePSA system and its ability to selectively adsorb one component from afeed gas mixture in preference to another, less readily adsorbablecomponent. Suitable adsorbent materials may include zeolitic molecularsieves, activated carbon, silica gel, activated alumina and the like.Zeolitic molecular sieve adsorbents are generally desirable for saidoxygen separation and recovery from air, with said 13 X adsorbent or 5Amolecular sieve being standard materials that can readily be employed inthe commercial practice of the prior art approaches improved as hereindisclosed and claimed.

It will be understood that various operating conditions can be employedin the practice of the invention, depending upon the particularseparation being carried out, the purity level desired, the adsorbentmaterial employed, and the like. It has been found, however,particularly with respect to the separation and recovery of oxygen fromair, that an upper adsorption pressure of from about 40 to about 60psig, preferably about 45 to about 55 psig, is desirable. Desorption isconventionally at about atmospheric pressure, but other, higher or lowerdesorption pressures can also be employed in particular applications.The invention enables the overall cycle time to be desirably minimized,with cycle times of from about 140 to about 180 seconds being feasiblein various embodiments, particularly in 4-bed systems, while somewhatlonger times may be required in embodiments such as illustrated in TableII wherein a 5-bed system was employed with two beds in adsorption atany given time and in which an increase in recovery was obtainable ascompared with the 4-bed system illustrated in Table I. In general,oxygen product recovery in air separation applications of the inventionare readily obtainable within the range of from about 50% go about 60%,typically from about 53% to about 55%.

The invention will thus be seen to satisfy the desire in the are forimprovements in the PSA technology as applied to various gas separationoperations, such as the separation and recovery of oxygen from air. Thesimultaneous cycle steps of the invention thus enable increasedabsorbent productivity to be achieved, providing increased bed capacity,while product recovery improvements averaging about 5 to 6% can beobtained as compared with the commercial 3-bed PSA process. Theprocessing cycles of the invention advantageously employ provide purgesteps shorter in time than the time provided for the actual purge of thebed, with the overall cycle times being minimized without degradation ofproduct effluent purity. The invention thus enhances the feasibility ofapplying PSA technology in practical, commercial gas separationoperations in a more efficient, effective manner than was heretoforepossible utilizing the PSA technology as developed heretofore in theart.

We claim:
 1. In a pressure swing adsorption process for the separationand recovery of a less readily adsorbable component of a feed gasmixture in an adsorption system capable of selectively adsorbing a morereadily adsorbable component from said gas mixture, the adsorptionsystem containing at least three adsorbent beds, each of whichundergoes, on a cyclic basis, a processing sequence that includes (1)adsorption with discharge of the less readily adsorbable component asproduct effluent from the product end of the bed, (2) cocurrentdepressurization with release of void space gas from the product end ofthe bed and the passage of said gas to other beds in the system forpressure equalization and for provide purge purposes, (3) countercurrentdepressurization to lower desorption pressure with release of said morereadily adsorbable component from the feed end of the bed; (4) purge atsaid lower desorption pressure, (5) partial repressurization by pressureequalization with void space gas from other beds, and (6) furtherrepressurization to upper adsorption pressure, the improvementcomprising:(a) passing void space gas released from the bed duringcocurrent depressurization thereof from said upper adsorption pressureto an upper intermediate pressure to another bed in the system, saidother bed thereby being partially repressurized to said upperintermediate pressure; (b) passing additional void space gas releasedfrom the bed during further cocurrent depressurization thereof from saidupper intermediate pressure to an intermediate level to a different bedin the system, said additional gas comprising a purge gas for saiddifferent bed; (c) passing additional void space gas released from thebed during still further cocurrent depressurization thereof from saidintermediate pressure level to a lower intermediate pressure to anotherbed in the system initially at a lower pressure for pressureequalization therebetween at said lower intermediate pressure, whilesimultaneously countercurrently depressurizing said bed by the dischargeof gas from the feed end of said bed being depressurized; (d)discontinuing the passage of said void space gas to said bed initiallyat lower pressure upon said bed being depressurized and said bedinitially at lower pressure reaching said lower intermediate pressure;(e) further continuing to contercurrently depressurize said bed beingdepressurized, after completion of said step (c), down to the lowerdesorption pressure of said bed; and (f) discharing the less readilyadsorbable component as product effluent from the product end of the bedsimultaneously with the passage of the feed gas mixture to the feed endthereof for the repressurization of said partially repressurized bed tosaid upper adsorption pressure; and (g) repeating said steps (a)-(f) asthe cyclic operation is continued with additional quantities of feed gaswithout passing feed gas to the bed for adsorption at the upperadsorption pressure for discharge of the less readily adsorbablecomponent therefrom at constant upper adsorption pressure prior tocommencing cocurrent depressurization to said upper intermediatepressure level.
 2. The process of claim 1 in which said system comprisesfour adsorbent beds.
 3. The process of claim 1 in which said systemcomprises from five to seven adsorbent beds.
 4. The process of claim 1in which said feed gas mixture comprises air, said less readilyadsorbable component comprises oxygen, and said more readily adsorbablecomponent comprises nitrogen.
 5. The process of claim 4 in which saidsystem comprises four adsorbent beds.
 6. The process of claim 5 in whichsaid upper adsorption pressure is from about 40 to about 60 psig.
 7. Theprocess of claim 6 in which said upper adsorption pressure is about 45to about 55 psig.
 8. The process of claim 7 in which said overall cycletime is from about 140 to about 180 seconds.
 9. The process of claim 8in which said oxygen product recovery is from about 53% to about 55%.10. The process of claim 5 in which oxygen product recovery is fromabout 50% to about 60%.
 11. The process of claim 4 in which said systemcomprises from five to seven beds.
 12. The process of claim 11 in whichsaid upper adsorption pressure is from about 40 to about 60 psig. 13.The process of claim 12 in which said upper adsorption pressure is fromabout 45 to about 55 psig.
 14. The process of claim 13 in which twoadsorbent beds are on the adsorption step, in overlapping sequence, atany given time in the cycle, oxygen product recovery being from about50% to about 60%.
 15. The process of claim 14 in which said oxygenproduct recovery is from about 53% to about 55%.
 16. The process ofclaim 11 in which said feed gas mixture comprises air, said less readilyadsorbable component comprises oxygen and said more readily adsorbablecomponent comprises nitrogen.
 17. The process of claim 16 in which saidsystem comprises four adsorbent beds, in which said upper adsorptionpressure is from about 40 to about 60 psig, said product recovery isfrom about 50% to about 60T, and said overall cycle time is from about140 to about 180 seconds.
 18. The process of claim 16 in which saidsystem comprises from five to seven adsorbent beds, with two beds on theadsorption step, in overlapping sequence, at any given time in thecycle, oxygen product recovery being from about 50% to about 60%.